Tree leaves and pods form a natural part of the diet of many ruminant species and have been used traditionally as sources of forage for domesticated livestock in Asia, Africa and the Pacific (Skerman 1977, NAS 1979, Le Houerou 1980a). Although not all forage trees are legumes, more than 200 species of leguminous trees are reported to be used for forage, with most species being tropical or subtropical in origin. The most commonly used species come from the genera Acacia, Albizia, Calliandra, Desmanthus, Desmodium, Gliricidia, Leucaena, Prosopis and Sesbania (Brewbaker 1986). Compared with herbaceous legumes, tree and shrub legumes have received relatively little attention in the search for productive and persistent forage sources for the tropics (NAS 1979). In Australia, most research has centred on native tree species which are generally of low nutritive value (Wilson and Harrington 1980). The recent success with Leucaena as a new high quality forage species for northern Australia (Jones 1979) has prompted a search for other legume tree and shrub species suitable for introduction to other Australian grazing systems.

Tree legumes must have both desirable agronomic characteristics and high nutritive value to be useful as forages. The nutritive value of a feed is determined by its ability to provide the nutrients required by an animal for its maintenance, growth and reproduction. Tree legumes have been mostly used as feeds for ruminants, although there are some reports of their inclusion in the diets of non-ruminants (pigs and poultry). The leaves, stems and fruits may be used either as a complete feed or as a supplement to other feeds. In some species, a major limitation to the use of one or more of these components is the presence of toxic and/or anti-nutritive factors.

This section reviews the nutritive value of forage trees when used as a single feed. Later sections examine the use of tree legume foliage as a supplementary feed and the effects of anti-nutritive factors found in tree leaves on their nutritive value.

Nutritive value is a function of the feed intake (FI) and the efficiency of extraction of nutrients from the feed during digestion (digestibility). Feeds of high nutritive value promote high levels of production (liveweight gain). Feed intake in ruminants consuming fibrous forages is primarily determined by the level of rumen fill, which in turn, is directly related to the rate of digestion and passage of fibrous particles from the rumen. Voluntary consumption of feed may also be modified by animal preference, some feeds being eaten in smaller or larger amounts than predicted by digestibility (D) (Egan et al. 1986). The acceptance or edibility (palatability) of a feed has been related to both physical characteristics (hairiness and bulk density) and the presence of compounds which may affect taste and appetite (volatile oils, tannins and soluble carbohydrates).

The productivity of ruminants is closely associated with the capacity of a feed to promote effective microbial fermentation in the rumen and to supply the quantities and balances of nutrients required by the animal tissues for different productive states.

There is no simple predictor of the quality of tree legume foliage. Chemical composition alone is an inadequate indicator of nutritive value since the availability of nutrients from forages is variable. Digestible dry matter (or energy) intake (FI x D) may also be a poor predictor of potential productivity since the composition of nutrients absorbed is not described. Modern concepts of feed evaluation require that quality be assessed in terms of the capacity of a feed to supply nutrients in proportions balanced to meet particular productive functions (Leng 1986). The nutritive value of feeds should be ranked on the following characteristics:

· voluntary consumption potential,

· potential digestibility and ability to support high rates of fermentative digestion,

Much of the considerable information now available on the chemical composition of tree foliage (Gohl 1981, Le Houerou 1980a) is from proximate analysis (total N. ether extract, crude fibre, nitrogen free extractives) and is of limited value as a predictor of nutritive value. Analyses based on detergent extraction are more useful since plant dry matter is separated into a completely digestible fraction (neutral detergent solubles (NDS)) representing cell contents, and a partially digestible fraction (neutral detergent fibre (NDF)) representing plant cell walls.

Element (N, P, S, etc.) composition provides values which can be compared with animal requirements. However, whilst values less than predicted requirements are indicative of deficiency, values greater than prescribed are not necessarily indicative of sufficiency. Not all elements are fully available for use by the microbial population in the rumen or for absorption in the intestines. Some examples of this will be presented. Table 4.1.1 shows that the chemical composition of a selected range of tree legume species varies with soil type (location), plant part (leaf, stem, pods), age of leaf and season. Further variability in composition is introduced when subjective selection of the 'edible' fractions of tree foliage is made. Edible fractions tend to contain higher stem contents and to be of lower nutritive value than leaves alone.

Proteins and tannins

The protein content of forage tree legume leaves (12-30%) is usually high compared with that of mature grasses (3-10%). The proteins are digested in the rumen to provide ammonia and amino acids for microbial protein synthesis. Microbial cells then pass to the small intestine, providing the major source of absorbed amino acids for the ruminant. In some cases, feed proteins may escape digestion (bypass proteins) in the rumen and provide additional protein for absorption in the small intestine. The microbial population in the rumen requires a minimum level of ammonia (70 mg N/l) to support optimum activity; lower values are associated with decreased microbial activity (digestion) and are indicative of nitrogen deficiency. Feeds containing less than 1.3% N (8% crude protein) are considered deficient as they cannot provide the minimum ammonia levels required. All forage tree legumes have N contents higher than this value, and may be judged adequate in protein. However, tannins found in some tree legume leaves form complexes with plant proteins which decrease their rate of digestion (degradability) in the rumen, thereby decreasing rumen ammonia concentrations and increasing the amount of plant protein bypassing the rumen. Where the tannin-protein complexes are dissociated in the low pH of the abomasum, an additional source of protein is made available for absorption by the animal. In other cases, the tannins protect the proteins from digestion even in the small intestine.

Tannins may therefore have a beneficial effect (increasing bypass protein or decreasing ammonia loss) or a detrimental effect (depressing palatability, decreasing rumen ammonia, decreasing post-ruminal protein absorption) on protein availability. It is clear that the interpretation of the nutritional value of protein in forage trees requires information on the nature and action of tannins. The proteins in the leaves of species which do not have tannins (Albizia lebbeck, Enterlobium cyclocarpum, Albizia saman and Sesbania spp.) will be rapidly degraded in the rumen, providing high levels of rumen ammonia, much of which will ultimately be wasted by excretion as urinary urea. Such feeds provide N in a similar way to urea. Species which contain some tannins will therefore provide both degradable and undegraded rumen N and will be more effective sources of supplemental N for ruminants. Nevertheless, the significance of tannins in tree legume forage is poorly understood, with low concentrations being beneficial and high concentrations detrimental. It is also likely that not all tannins act similarly; this area requires further study.

Macro and trace element content

Table 4.1.2 shows some values for the concentrations of elements in a range of forage trees. There is little information available on the trace elements (Cu. Mn, Zn, Co, I) and only fragmentary data on the macro-elements. Sulphur (S) in plant material is mainly found in the form of sulphur amino acids and is required, together with N. for microbial protein synthesis in the rumen. Concentrations greater than 1.5 g/kg dry matter or N:S ratios less than 15:1 are considered adequate. However, where protein digestion in the rumen is limited by complexing with tannins, S rather than N may become the limiting factor in microbial protein synthesis. From the data available, most species appear to meet the S requirements of ruminants, with the possible exception of the Acacia spp.

The minimum requirement of ruminants for phosphorus (P) varies from 1.2 to 2.4 g/kg feed dry matter depending on physiological function. Forage tree leaves generally have high P concentrations. Mulga (Acacia aneura) is an exception with low P contents and responses of sheep to P (+ molasses) supplementation have been reported (McMeniman and Little 1974). Calcium (Ca) is closely associated with P metabolism in the formation of bone, and a Ca:P ratio of 2:1 is usually recommended for ruminant diets. Ca is rarely limiting in forage diets and the same is true for forage trees (Table 4.1.2). However, high concentrations of oxalic acid in leaves may decrease the availability of Ca during digestion. Gartner and Hurwood (1976) have suggested that high oxalate levels in mulga affect Ca metabolism in sheep. Magnesium (Mg) and potassium (K) are found in excess of requirements in tree leaves and are seldom a limiting dietary factor in ruminants.

Although sodium (Na) deficiency has been recorded in cattle grazing tropical pastures, short term deficiencies are rare. Ruminants effectively conserve tissue Na by recycling it through the rumen. The recommended requirement for Na in ruminant diets is 0.7 g/kg dry matter. Some tree species appear to be marginal in Na, but deficiencies are probably not common as forage tree leaves usually form only part of a ruminant's diet. Deficiencies of minerals other than S and P appear to be unlikely, although leucaena is reported to be low in both Na and I.

When compared with information on chemical composition, there is less known about the feeding value of tree foliage for stock. The digestibility of plant material in the rumen is related to the proportion and lignification of plant cell walls (NDF). Tree forages with a low NDF content (20-35%) are usually of high digestibility and species with high lignin contents are often of low digestibility. Bamualim et al. (1980) showed that the lignin content of tree foliage was negatively correlated (r = -0.92) with feed digestibility in nylon bags. Stems have higher lignin contents than leaves, and are less digestible. It may therefore be predicted, from their high NDF and lignin contents (Table 4.1.1), that Acacia spp. and Albizia chinensis will be of low digestibility. More information on the NDF, ADF, lignin and tannin content of tree foliages is needed if a comprehensive assessment of their nutritive value is to be made.

In vitro digestion techniques (Tilley and Terry 1963, McLeod and Minson 1978) provide comparative estimates of dry matter digestibility between feeds. These values may be used to rank the quality of feeds but usually underestimate measured in vivo digestibility. New techniques which measure the rates of feed digestion in nylon bags suspended in the rumen (in sacco) (Mehrez and Orskov 1977, Ffoulkes 1986) can also be used to rank feeds. This technique has the advantage that the rates of digestion of different feed components (protein and starch) may also be calculated. In sacco digestibility usually overestimates in vivo digestibility. Table 4.1.3 shows values for in vitro, in sacco and in vivo estimates of tree legume digestibility, and for the intake of foliage by different ruminant species. These data were collected under a range of conditions, and only general conclusions can be drawn about the comparative value of the different species.

In vitro (IVD) and in sacco (ISD) estimates of feed digestibility generally ranked feeds in the same order, but these values were not useful predictors of either in vivo digestibility (DMD) or voluntary feed intake (VFI). For this reason, these techniques provide only qualitative data on feed nutritive value. The usefulness of IVD and ISD, as estimates of quality, is to predict DMD, which is closely related to VFI for tropical grasses (Minson 1982). Data in Table 4.1.3 suggest that the same predictions cannot be made for forage trees. For example, similar values (66-68%) were found for the IVD of tagasaste (Chamaecytisus palmensis)and gliricidia, whereas in vivo digestibilities were found to be 76 and 55% respectively. Intake values were similar. Similarly, DMD was not a guide to intake of Albizia lebbeck, where sheep consumed significantly more fallen leaf (DMD 43%) than fresh leaf (DMD 64%). Conversely, goats appeared to consume similar amounts of Leucaena (35.6 g/kg/liveweight (LW)) and gliricidia (32.6 g/kg LW) despite large differences in digestibility (68.0 and 56.3% respectively).

These results suggest that factors other than the rate of digestion in the rumen determine the voluntary intake of tree foliage by ruminants. Low intakes associated with high feed digestibilities may be related to the presence of compounds which are appetite depressants (tannins, alkaloids, etc.). High feed intakes and low feed digestibilities may be related to rapid rates of passage of feed through the rumen, such as when the small leaflets of pinnate leaves are being consumed. Since there are no known techniques which predict palatability and intake, the nutritive value (VFI x DMD) of forage tree species can only be accurately determined by feeding trials. Feeding trials have the added advantage of also providing information on animal health and productivity (liveweight gain).

The screening of forage trees for nutritive value by qualitative methods may therefore lead to some erroneous conclusions if not supported by feeding trials. Tree legumes of low digestibility and high palatability would be rejected. The form in which the leaves are fed (fresh, wilted or dry) is also known to affect both intake and digestibility in some species (Palmer and Schlink 1992). As will be discussed later, the problems of low digestibility (and intake) may be partially overcome by supplementary feeding or by combination with other feeds.

While all species contained high concentrations of proteins (Table 4.1.1), the N degradability of protein, as estimated by in sacco methods, varied considerably. High degradability (>78%) was found in all species which did not contain tannins, while most tannin-containing species were of low degradability (<39%). Exceptions were Codariocalyx gyroides, gliricidia and Leucaena which showed moderate degradability (64-84%) yet contained 3-7% tannins. The high intakes of the latter two species suggest that tannins, in these examples, did not reduce palatability. There is little known about the nature and chemistry of tannins in forage trees. It seems that not all tannins decrease protein degradability in the rumen.

Ahn et al. (1989) have shown that the drying of tree legume leaf decreases tannin content and, in the case of gliricidia and Tipuana tipu, removes all tannin. In most species, drying decreased tannin content and increased N degradability. Drying may be a practical means of manipulating protein availability from forage tree legumes. In contrast, the decreased tannin content after drying was associated with a decreased in sacco N digestibility for T. tipu while there was no significant effect on the degradability of gliricidia. Our understanding of these effects is incomplete.

Some tree legumes contain anti-nutritive factors which adversely affect nutritive value. For this reason, depending on the species, tree legume foliage may be of lower nutritive value as a sole feed than as a supplement to other feeds. The significance of secondary plant compounds becomes more evident when tree foliage is the only feed consumed.

Acacia species are generally of low nutritive value, and as a sole feed are little better than a maintenance feed for stock. Mulga (Acacia aneura)has received considerable research attention in Australia. Its nutritive value may be greatly increased by the provision of specific supplements. McMeniman and Little (1974) first demonstrated that supplementation of mulga with P in molasses increased wool growth in sheep. McMeniman (1976) also showed that sheep on mulga responded to the addition of urea to their diet, even though the diet contained more than the minimum level of crude protein. It was subsequently shown that sheep on these diets were also responding to the additional S in molasses (Hoey et al. 1976, Gartner and Niven 1978). Digestibility was only slightly increased by the supplements but voluntary consumption of mulga was increased by 150%.

Pritchard et al. (1988) showed that the feeding of polyethylene glycol (PEG) to sheep fed mulga markedly increased feed intake, weight gain and wool growth. The low quality of mulga is therefore related to its high content of condensed tannins and their capacity to bind feed proteins. These proteins are poorly digested in the rumen and appear also to be indigestible in the intestines. Consequently, sheep consuming mulga have low rumen ammonia and S levels, which can be corrected by S supplementation. The addition of PEG preferentially binds the tannins thereby making plant proteins more available for digestion. The increased digestion rate stimulates feed intake and changes mulga from a maintenance ration to one on which sheep can grow. Sulphur supplements to the drinking water are sufficient to produce this response. These findings are relevant to other Acacia species of low nutritive value.

Research with leucaena has resulted in the discovery of rumen bacteria capable of degrading 3 hydroxy-4(1H)-pyridone (DHP). Inoculation of cattle with the bacteria (see Section 4.5) increases the intake and productivity of cattle grazing this tree legume in Australia. There are prospects for isolating other bacteria with beneficial functions from the rumen which may be used as an inoculum to animals to offset the detrimental effects of tannins and other secondary plant compounds.

New problems will arise with each new species that shows agronomic promise, and careful evaluation of the nutritive value of each introduction needs to be made.

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McGowan, A.A., Robinson, I. and Moate, P. (1988) Comparison of liveweight gain and mineral metabolism of sheep fed pasture of Tagasaste. Proceedings of the Australian Society of Animal Production 17, 230-233.

McLeod, M.N. (1973) The digestibility and nitrogen, phosphorus and ash contents of the leaves of some Australian trees and shrubs. Australian Journal of Experimental Agriculture and Animal Husbandry 13, 245-250.

McLeod, M.N. and Minson, D.J. (1978) The accuracy of the pepsin-cellulase technique for estimating the dry matter digestibility in vivo of grasses and legumes. Animal Feed Science and Technology 3, 277-287.